Ever since setting up broadband at home, from time to time (on a daily basis) the internet connection would cut out for no more than 20 seconds.

We have three computers connected to the wireless here, and when these random cut outs occur we are all still connected to the router (Local only).

At first I put this problem down to just a bad choice of ISP, but then one of the house-mates went away for a week, and during that time we experianced no internet cut outs at all, I had my laptop on for at least 4 days in a row non-stop monitoring drops and there were none.

As soon as our 3rd housemate returned home, we started having these drops again.

So my question is: what are the possible reasons for these cut outs? Could it simply be down to using more bandwidth when the 3 of us are all connected, or are there any known (perhaps malicious) programs that may cause such occurances? We have an unlimited internet package, but I am pretty sure it is something to do with this 3rd pc, as at times when one of the other 2 are not functioning, we stil have the drops.